GTP-DEPENDENT FINDING OF THE ANTIPROLIFERATIVE AGENT DIDEMNIN TO ELONGATION-FACTOR 1-ALPHA

The marine natural product, didemnin B, is a 7-amino acid, cyclic deps ipeptide that inhibits G(1) cell cycle progression at nanomolar concen trations by undefined mechanisms. It has been reported to exhibit immu nosuppressive activities in animals and is undergoing clinical trials as a potential antineoplastic drug. In addition, at higher concentrati ons, didemnin B has been shown to inhibit in vivo and in vitro protein synthesis. However, the mechanisms by which inhibition is achieved ar e unknown. To investigate didemnin's various modes of action, an affin ity column was synthesized and used to purify didemnin-binding protein s. The major retained protein was the 49-kDa guanine nucleotide-bindin g elongation factor, EF-1 alpha, which was identified by peptide seque nce analysis, Moreover, didemnin binds EF-1 alpha only in the presence of GTP but does not inhibit the GTPase activity of EF-1 alpha. Theref ore, EF-1 alpha is likely to be the intracellular target responsible f or didemnin B's ability to inhibit protein synthesis. Furthermore, thi s specificity of didemnin affinity for the GTP-bound conformation of a guanine nucleotide-binding protein with homology to the Ras superfami ly suggests a possible mode of action for didemnin's antiproliferative activity.